I think we may be missing the point here. I think the OP is concerned about the reliability of the oscillator, not its precise frequency. The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all.

If you are concerned about reliability of oscillator due to load capacitors - you badly messed-up something. Usually you just open datasheet of the crystal and know load capacitor values. You want to measure if you have either quality concerns of crystal, doubt about drive strength or want to check/tune frequency.

I think we may be missing the point here. I think the OP is concerned about the reliability of the oscillator, not its precise frequency. The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all. I think you would select the capacitors by starting with the datasheets and a little math, then doing final testing with various values and testing both the crystal voltage to make sure it isn't too high and then the startup characteristics over the range of temperatures you expect the device to be subjected to. If you want to measure and view those things, you need a probe such as the one I mentioned--and even that is going to have at least a small effect.

This isn't tuning a transmitter. How accurate do you thing the average internal MCU oscillator circuit is anyhow, even with a proper crystal?

If the OP is worried about the reliability of the oscillator, surely "the proof of the pudding is in the eating"?The thing has been out in the market for some time --have they had complaints about the reliability with the later crystal type?

In any case, crystal manufacturers usually give suggested values for components like the "load" capacitors.If they suggest a different value--- change to that value!

As I pointed out earlier in a more diplomatic manner, the OP doesn't seem to have the first clue about what he/she is trying to achieve.It certainly looked like they are concerned with frequency stability, but the fact that they thought 400Hz & 1.2kHz were possible outcomes raises a serious doubt.

Ultimately, as you pointed out, it is questionable how accurate the thing will be, or needs to be, for that matter.By & large crystal oscillators are not particularly critical, & millions of them operate reliably every second of the day.

The OP made it clear that it was a "noob" question, and they were looking for some advice. It sounds like they've inherited a design and they want to learn how to ensure it is still appropriate with various component/supplier changes. Seems like a reasonable question, and we've all had to learn things like this ourselves in the past. No need to get rude.

One thing that can be done is to have the xtal manufacturer verify the design will work over a wide range of evironmental conditions. Need to supply the relevant part of the schematic and samples of the circuit.

It is something done for the automotive industry, I never knew this until I join my current company and the electronics engineers told me it is standard practise for xtals used in automotive designs.No idea how much it costs.

I've read other such papers. Putting a current probe on the xtal is inconvenient at best in a repair situation or when evaluating an already assembled circuit. The same goes for bodging in test resistors, although to be fair, since the OP is arguably in the design/redesign phase, this might not be an unreasonable way to test. As far as grounding, are you proposing not using the probe ground lead? If not, where do you propose connecting it? Some XOs, like the last one I worked on, are in isolated circuits. Every circuit is different and IMO the easiest, quickest way of measuring startup and drive level characteristics would be an active differential probe right across the crystal. I'm not sure where your "bizarre" comment comes from--a single ended probe connected to a regular oscilloscope has a ground lead that when connected will ground that part of the circuit. Now it may be the case that many XOs can be measured easily with a single-end low capacitance probe, but I assure you that some can't.

As far as grounding, are you proposing not using the probe ground lead? If not, where do you propose connecting it? Some XOs, like the last one I worked on, are in isolated circuits. Every circuit is different and IMO the easiest, quickest way of measuring startup and drive level characteristics would be an active differential probe right across the crystal. I'm not sure where your "bizarre" comment comes from--a single ended probe connected to a regular oscilloscope has a ground lead that when connected will ground that part of the circuit. Now it may be the case that many XOs can be measured easily with a single-end low capacitance probe, but I assure you that some can't.

Any XO circuit needs power, it has positive supply rail and negative supply rail which usually is considered as ground in unipolar circuits. So any XO already has ground where it's signals are referenced-to. Connect ground clip of your probe there. Saying "you generally can't ground either side of an XO circuit" does not even makes sense or we got some strange language barrier problem here. [edit] By grounding any side of XO I understand - short it's output or input to ground. Obviously such suggestion is bizarre to me.

Most of the oscillators are either Colpitts or cmos inverter topology, boths has grounds, both can be measured using fet probe. No need for fancy FET differential probes I am not sure even exist. Could you show pointers to differential probe you are talking about?

One thing that can be done is to have the xtal manufacturer verify the design will work over a wide range of evironmental conditions. Need to supply the relevant part of the schematic and samples of the circuit.

The most common problem is the gain element used for the oscillator. CMOS gate oscillators are especially problematical because MOSFET transconductance on a digital process is not well controlled and batches of microcontrollers have been occasionally released which would not start at low temperatures.

This makes oscillators using discrete transistors more reliable but it is a good idea to test operation beyond the full temperature range.

As far as grounding, are you proposing not using the probe ground lead? If not, where do you propose connecting it? Some XOs, like the last one I worked on, are in isolated circuits. Every circuit is different and IMO the easiest, quickest way of measuring startup and drive level characteristics would be an active differential probe right across the crystal. I'm not sure where your "bizarre" comment comes from--a single ended probe connected to a regular oscilloscope has a ground lead that when connected will ground that part of the circuit. Now it may be the case that many XOs can be measured easily with a single-end low capacitance probe, but I assure you that some can't.

Any XO circuit needs power, it has positive supply rail and negative supply rail which usually is considered as ground in unipolar circuits. So any XO already has ground where it's signals are referenced-to. Connect ground clip of your probe there. Saying "you generally can't ground either side of an XO circuit" does not even makes sense or we got some strange language barrier problem here. [edit] By grounding any side of XO I understand - short it's output or input to ground. Obviously such suggestion is bizarre to me.

Most of the oscillators are either Colpitts or cmos inverter topology, boths has grounds, both can be measured using fet probe. No need for fancy FET differential probes I am not sure even exist. Could you show pointers to differential probe you are talking about?

I agree that any XO circuit needs positive and negative supply and if you wish you can assert that convention implies the negative to be "ground". However, that may be isolated from or at a different potential than other grounds, including the case of your DUT or the shell of the oscilloscope BNC connector. Isolation isn't all that rare, especially in test instruments. In one particular case, I was able to see the XO signal simply using a single 100x 100M probe with no ground lead. However, to accurately measure the crystal drive voltage, you have to either know the particulars of the circuit and plan it out carefully--or use a low capacitance active differential probe.

I've been stalking them on eBay, but they still go for big bucks unless you buy them as-is, which means someone probably burned it out--they're quite sensitive.

It is widely known that differential probes exist. I did ask you differential FET probe. Hint: one with FET transistor input. TEK differential probes (P624*) are *not* FET probes because they have differential 200k input resistance and 100k common mode resistance (attach) - as soon as you connect such to your oscillator, it (oscillator) will stop operation.

I've been stalking them on eBay, but they still go for big bucks unless you buy them as-is, which means someone probably burned it out--they're quite sensitive.

It is widely known that differential probes exist. I did ask you differential FET probe. Hint: one with FET transistor input. TEK differential probes (P624*) are *not* FET probes because they have differential 200k input resistance and 100k common mode resistance (attach) - as soon as you connect such to your oscillator, it (oscillator) will stop operation.

I didn't say FET, although I do believe the P6247 does use them, just not in the high-impedance direct input configuration that typical single-ended active-FET probe might have. I'm fully aware of the input impedances of these probes. I am also aware of the significant voltage limitations, which is why the eBay ones may be commonly burned out. I don't have one to try yet, but I don't think they will kill a typical XO because 200K is still much higher than the impedance of the small input capacitance at most XO frequencies I'm worried about. It is also much less than the typical impedance of the crystal itself. I think it would work fine for a ground-referenced or truly isolated circuit, but the common-mode voltage limitation would prevent using it on a circuit that was offset from ground unless you have a separate isolator and float the probe power supply.

As for the existence of a "high-impedance differential active FET" probe, I can't say I know of one. I did have the idea of taking two of the devices that Kean posted and summing the outputs. And that is what you would essentially need to build such a probe--a dual input active-FET and a summing circuit. I'd be surprised if someone somewhere hasn't already made such a thing, but as I said, I've not seen one. I suspect that it would not be easy to achieve a decent CMRR.

200K is still much higher than the impedance of the small input capacitance at most XO frequencies

Before we continue this debate, you better show ANY mention of using differential probes for XO measurements in papers from reputable sources, preferably crystal/oscillator manufacturers. I did show Epson papers suggesting that only single FET probe and little brain cells needed to check XO.

200K is still much higher than the impedance of the small input capacitance at most XO frequencies

Before we continue this debate, you better show ANY mention of using differential probes for XO measurements in papers from reputable sources, preferably crystal/oscillator manufacturers. I did show Epson papers suggesting that only single FET probe and little brain cells needed to check XO.

Oh dear! Papers from reputable sources? I'm not that invested in this teapot tempest. And as I read it, the paper you cited directed the use of a FET probe, a current probe which has to be installed in a test circuit and some math. I know that method and I've read it before. I'm trying to substitute direct measurement across the xtal and knowledge of it's characteristics to avoid the current probe necessity in repair and analysis of the drive levels. If I happen to acquire a suitable probe, I will try it and report back. I'm not going to write a paper, but I'll let you know either way!

If I happen to acquire a suitable probe, I will try it and report back.

Good luck with that. Before you invest into probe - solder some 100k load resistors to various crystals and see how it goes. Start with 32KHz tuning fork crystal.

How did we go from discussing a 12MHz MCU crystal to a 32kHz low-power unit with an ESR that is probably 1000X higher? Yes, obviously I'd have to adapt my method--but it doesn't take much imagination to see a way to do it.

I think we may be missing the point here. I think the OP is concerned about the reliability of the oscillator, not its precise frequency. The load capacitors will change many characteristics, including the voltage across the crystal and the startup time, or if they are off enough, they may prevent reliable startup at all. I think you would select the capacitors by starting with the datasheets and a little math, then doing final testing with various values and testing both the crystal voltage to make sure it isn't too high and then the startup characteristics over the range of temperatures you expect the device to be subjected to. If you want to measure and view those things, you need a probe such as the one I mentioned--and even that is going to have at least a small effect.

This isn't tuning a transmitter. How accurate do you thing the average internal MCU oscillator circuit is anyhow, even with a proper crystal?

Yeah I think a number of replies are going way too complex for what I am trying to do.

I am not suggesting I am the best at electronics or testing anything, and the fact I missed the 'glaringly obvious' fact that xtals cant work at 400hz or 1.2khz when they are meant to be 12Mhz, I apologise. You are superior than me.Turns out I was looking too far out on the scope and had to 'zoom in' a bit, and the 12Mhz then came up fine. I did find it odd that it wasn't locking on to the 400hz very well, despite my trigger, and I guess it explains why.I don't do this type of work daily.

I have a goal I am trying to reach and I am doing the best I can to try and achieve that goal.

We have systems which are all running fine. The 12Mhz isn't in question really, the products work. I am just trying to determine if the brand/model of crystals we have currently are running the best they can based on the loading capacitors in the circuit. That is it.I know crystals work at quite a lot of values within reason, but then become problems when the temperature goes up or down, and other factors like that. Nearby noise etc.We have had some problems in the past when our product is used in particular environments where there are high noise devices, and our products can go a bit weird. I am just trying to work out if a factor like our loading caps on the particular brand of xtal we are running at the moment, is contributing to these edge cases.

IF working it out from the xtal datasheet, is it a case of "Capacitor Value = 2x CL - 2x Stray" ? or something close to that sort of formula?

You are actually on the right track. Datasheets only go so far. Also, you need both the XTAL datasheet and the MCU datasheet and you have to know approximately what the stray capacitance of the circuit board is (hint: keep the traces absolutely as short as possible...)

I'm not really an expert and I'm learning too--I just fix things and have run into an occasional issue with this. Some XO designs, especially MCUs, are not particularly robust. The two tests I think matter most are the negative resistance test, where you add a series resistance until the oscillator fails, and measuring the drive level. The idea is that you want the XO design to work even if a particular crystal is a bit high ESR or the MCU is a bit weak. However, you don't want to overdrive the crystal or it won't last and the output will suffer distortion and phase noise.

I don't know how many products you are dealing with, but specifics might help--including closeup photos of the actual board layout.

Without knowing for sure how much the probes will effect the result , the best way to test it is to write a little program to toggle a pin with the maximum frequency possible and measure there ... You can change then the oscillator components and see the tolerances margin . Ok , you could buy some good active probes , or other equipment , but you need a time to learn how to use them properly ...

the best way to test it is to write a little program to toggle a pin with the maximum frequency possible and measure there ...

Already mentioned that.

I already said that some microcontrollers have a gpio pin dedicated to outputting the oscilator so that you can see what freq the xtal is running at, you can probe this output with oscilloscope without affecting the xtal.You have to write some code to enable that pin.

How did we go from discussing a 12MHz MCU crystal to a 32kHz low-power unit with an ESR that is probably 1000X higher?

We discussed crystal oscillator measurements, suggested methods that work for most of the crystals. Fact that you push towards HF 12MHz crystal to make your invention of using differential probe work, is your problem. I already said that - good luck with that.

OP needs just one thing - ensure that load capacitors have right capacitance. Easy way is to measure frequency of XO w/o loading it much with probe. If you have option to output 1:1 or divided frequency to dedicated pin - then you can use 1:10 generic scope probe and call it a day, considering that your scope have good enough frequency counter. Otherwise you need to use dedicated counter. If there is no pin to output MCU clock - you better use FET probe to measure XO output. You need to check couple of devices/crystals and ensure that they fall into specified frequency tolerance, their "ppm specs".

Drive strength measurements are needed when you need to find lowest possible, still stable drive strength in low power application. Obviously it would be every real-time clock that uses low frequency crystal (32KHz). When you use HF crystals and saving 1mW is not important - just drive your crystal at max as whole industry do, that's it.

How did we go from discussing a 12MHz MCU crystal to a 32kHz low-power unit with an ESR that is probably 1000X higher?

We discussed crystal oscillator measurements, suggested methods that work for most of the crystals. Fact that you push towards HF 12MHz crystal to make your invention of using differential probe work, is your problem. I already said that - good luck with that.

OP needs just one thing - ensure that load capacitors have right capacitance. Easy way is to measure frequency of XO w/o loading it much with probe. If you have option to output 1:1 or divided frequency to dedicated pin - then you can use 1:10 generic scope probe and call it a day, considering that your scope have good enough frequency counter. Otherwise you need to use dedicated counter. If there is no pin to output MCU clock - you better use FET probe to measure XO output. You need to check couple of devices/crystals and ensure that they fall into specified frequency tolerance, their "ppm specs".

Drive strength measurements are needed when you need to find lowest possible, still stable drive strength in low power application. Obviously it would be every real-time clock that uses low frequency crystal (32KHz). When you use HF crystals and saving 1mW is not important - just drive your crystal at max as whole industry do, that's it.

The OP stated 12MHz and an MCU, IIRC. My ramblings henceforward reflected this, that is something we call context.

As for the rest of your statements, that is the exact thinking that has yielded products that work most of the time but sometimes they don't and nobody knows why. If you are in the least concerned about XO reliability, you have to do the stability and drive level tests. If you are stuck with a particular MCU and production changes, you may need to change the circuit or the crystal to get reliable operation, and if you can't get reliable operation with a particular MCU under test conditions, you may need to use an external oscillator. These are the things the OP needs to know, since he is clearly concerned with reliability beyond the "hey, it works OK" level.

The OP stated 12MHz and an MCU, IIRC. My ramblings henceforward reflected this, that is something we call context.

Context is "Scope and Probes - Measuring Crystal" - subject of this thread.

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If you are stuck with a particular MCU and production changes, you may need to change the circuit or the crystal to get reliable operation, and if you can't get reliable operation with a particular MCU under test conditions, you may need to use an external oscillator. These are the things the OP needs to know, since he is clearly concerned with reliability beyond the "hey, it works OK" level.

Right. Using unproven way of measuring crystal oscillators suggested by "expert" who can't find any 3rd party document of proposed method, nor confirm that he tested it himself is proper way of ensuring product reliability. Good luck with that.

You could use the differential probe to measure the supply current into the drive amplifier and calculate the crystal current it from there. Trying to measure it directly with a current shunt would be an interesting exercise.